The present invention relates to laser machining, and, in particular, to an apparatus and methods for characterizing the spot size of a laser beam incident upon a workpiece and the distance from the spot to a fiducial reference.
The size and shape of the spot at which a laser beam impinges upon a workpiece is of paramount importance in laser welding, cutting and related applications. In order to control such machining operations, the distance between the laser focus and the workpiece must be monitored accurately, and tight tolerances must typically be maintained in order for the rated performance of a particular laser to be maintained. More particularly, the position and orientation of a tool head relative to a workpiece must be monitored and controlled in order to carry out programmed instructions for manufacturing operations.
Schmidt (U.S. Pat. No. 5,340,962), which is incorporated herein by reference, describes some of the challenging features of laser cutting and welding processes that give rise to the need for automatic focus control. A constant separation or gap must be maintained between the nozzle tip and the part, even though the shape of the part being cut or welded may not exactly conform to the part program that guides the laser nozzle or other tool over the part. It may be the case that the part is either warped or else retained slightly out of the correct position, or the part may move or flex due to stress relief as it is being cut or welded. If the gap varies appreciably from the specified value, the focal point of the laser beam, located a small distance below the nozzle tip, will not be in the correct location relative to the part and the beam will be xe2x80x9cout of focus.xe2x80x9d When this occurs, unsatisfactory cuts or welds will usually result.
Automatic focus controls, also referred to as AFCs, typically utilize a small linear servo system which moves the lens/nozzle assembly in a direction parallel to the beam and nozzle under the control of a gap sensor which may use either a capacitor, an eddy current sensor, or some combination thereof, which senses the gap between the nozzle and the workpiece and commands the beam positioner to keep the gap constant when the workpiece is not in its expected position relative to the part program. This may be caused by the part""s motion while being laser cut or simply that the part is not exactly the same shape as called for in the part program.
Solutions known and practiced in the art for maintaining a constant gap include capacitive sensors (such as taught in the Schmidt patent), triangulation sensors, and the characterization of a probe beam through a dithered focal element (as taught in Barrett et al., U.S. Pat. No. 4,978,841). Capacitive sensors typically provide a measure of displacement between the tip of a copper nozzle and the conductive part undergoing laser machining by employing the capacitance between these two as the frequency determining element of an oscillator. Thus, the frequency of the oscillator is inversely proportional to the capacitance which is itself directly proportional to the gap distance, and the separation of the tip and part is sensed just below the nozzle tip. Other examples of such capacitive techniques include Schmall (U.S. Pat. No. 4,298,784) which measures the capacitance as well as an inductance change as a function of the distance from the workpiece to the tool and the lateral position of the workpiece relative to the tool. Systems, however, that rely upon measurement of capacitance between the tool and the workpiece are not suitable for continuously monitoring the tool to workpiece distance while the tool operates if the tool creates an ionized plume as does a CO2 laser welding system. The plasma surrounding the nozzle tip during operation of the laser essentially shunts or short circuits the capacitance between the nozzle tip and the part so that monitoring the distance is impossible during periods when the beam is in operation.
The automatic laser focus system of Barrett involves directing a probe laser beam successively to three distinct spots on the workpiece, wobbling the beam while scanning the focussing lens in an axial direction, and searching for a maximum in the high frequency components of the light reflected from the workpiece.
Another method known in the art for measuring the gap is that of eddy current sensors. While, ideally, a winding or coil would be desirable in the nozzle itself in order to measure the gap in close proximity to the focal point of the laser beam, a metallic nozzle cannot readily be used in such a system since the nozzle itself would absorb much of the eddy current, rendering the sensor quite insensitive. Furthermore, nonmetallic nozzles often cannot withstand the intense heat of a weld plume.
It is thus desirable to provide a system and method for measuring the spot distance of a workpiece from the focus of a laser during operation of the laser tool and without dithering the focal element of a probe beam.
In accordance with a preferred embodiment of the present invention, there is provided a method for maintaining a specified distance between the focus of an optical assembly of a laser machining system and a workpiece having a surface disposed at a location relative to the focus of the optical assembly. The method has the steps of:
a. focusing a light output of a light source through the optical assembly substantially onto a spot on the surface of the workpiece;
b. deflecting light of the light source retro-reflected from the workpiece and through the optical assembly onto a distance-measuring apparatus;
c. measuring the distance to the spot on the surface of the workpiece relative to a fiducial reference; and
d. governing the location of the workpiece relative to the focus of the optical assembly based on the measured distance of the spot.
In accordance with alternate embodiments of the invention, the light source may be identical to a source of optical power used for at least one of welding and ablating the workpiece, and may have a spectrum substantially identical to the spectrum of the source of optical power used for welding or ablating the workpiece. The distance-measuring apparatus may be a conoscopic conoscopic holography apparatus, an autofocus apparatus, or a Shack-Hartmann wavefront sensor.